Prerequisites
- Functions and failure modes identified in the DFMEA risksheet (see Identify Functions and Failure Modes)
- Root causes documented at Level 3 of the hierarchy
Understanding the RPN Formula
The Risk Priority Number is the product of three independent ratings: RPN = Severity (S) x Occurrence (O) x Detection (D)
Severity is assigned at Level 2 (per failure mode) because it describes the effect’s impact. Occurrence and Detection are assigned at Level 3 (per cause) because they depend on the specific failure mechanism.
Step 1: Assign Severity (S)
Severity rates the impact of the failure mode effect on the patient, user, or device:- Click the Severity column (
fmSeverity) at the failure mode level (Level 2) - Select a severity rating:
| S Value | Label | Risk Qualifier | Color | Guidance |
|---|---|---|---|---|
| 1 | Negligible | Acceptable | Green (#58FF59) | No safety impact; cosmetic or minor functional issue |
| 2 | Low | Tolerable | Light Green (#ADFF59) | Tolerable under standard monitoring conditions |
| 3 | Moderate | Undesirable | Yellow (#FFFF59) | Requires verification or mitigation plan |
| 4 | High | Unacceptable | Orange (#FFB359) | Unacceptable without corrective actions |
| 5 | Critical | Intolerable | Red (#FF5959) | Intolerable until fully mitigated |
Step 2: Assign Pre-Mitigation Occurrence (O)
Occurrence rates how frequently the failure mode cause is expected to happen before any mitigation:- Click the Occurrence column (
premitigationFMOccurrence) at the cause level (Level 3) - Select an occurrence rating:
| O Value | Label | Probability Range | Guidance |
|---|---|---|---|
| 0 | Unanalyzed | — | Default; indicates analysis is incomplete |
| 1 | Never | < 5% | Failure mechanism is extremely unlikely |
| 2 | Seldom | 5-25% | Failure may occur in rare circumstances |
| 3 | Sometimes | 25-75% | Failure occurs occasionally during normal use |
| 4 | Often | 75-95% | Failure is expected to occur frequently |
| 5 | Always | > 95% | Failure is virtually certain |
Step 3: Assign Pre-Mitigation Detection (D)
Detection rates how likely the failure mode is to be caught before it reaches the user or patient:- Click the Detection column (
premitigationDetection) at the cause level (Level 3) - Select a detection rating:
| D Value | Label | Probability Range | Guidance |
|---|---|---|---|
| 0 | Unanalyzed | — | Default; indicates analysis is incomplete |
| 1 | Never | < 5% | Failure is almost never caught by current controls |
| 2 | Seldom | 5-25% | Failure may occasionally be caught |
| 3 | Sometimes | 25-75% | Moderate chance of detection |
| 4 | Often | 75-95% | Failure is usually caught by testing or inspection |
| 5 | Always | > 95% | Failure is almost always caught before reaching user |
In this solution, a higher Detection value means better detection capability. This differs from some traditional FMEA conventions (such as AIAG) where the scale is inverted (1 = almost certain detection, 10 = no detection). Higher D here reduces the concern, but since it multiplies into RPN, it actually increases the RPN number. Interpret RPN values accordingly.
Step 4: Review the Pre-Mitigation RPN
The risksheet automatically computes the pre-mitigation RPN: RPN = S x O x D The formula (commonRpn) multiplies the three factors and returns null if any factor is missing or zero.
RPN Color Coding
The RPN column and row header are color-coded based on the computed value:| RPN Range | Color | Risk Level | Typical Action |
|---|---|---|---|
| 1-10 | Green (rpn1) | Low risk | Monitor; no immediate action required |
| 11-30 | Amber (rpn2) | Medium risk | Evaluate and plan mitigation actions |
| > 30 | Red (rpn3) | High risk | Immediate mitigation actions required |
Step 5: Prioritize Failure Modes
After scoring all failure modes, review the RPN values to prioritize your mitigation efforts:- Sort by RPN column (descending) to see the highest-risk items first
- Focus on failure modes with RPN > 30 (red) for immediate action
- Review all failure modes with RPN 11-30 (amber) for planned mitigation
| Priority | RPN Range | Action |
|---|---|---|
| Immediate | > 30 | Assign mitigation actions now — see Add DFMEA Risk Mitigation Actions |
| Planned | 11-30 | Schedule mitigation actions for the current development phase |
| Monitor | 1-10 | Document rationale for acceptance; review at design milestones |
What Happens Next
- High-RPN items: Assign mitigation actions to reduce occurrence and/or improve detection — see Add DFMEA Risk Mitigation Actions
- Post-mitigation scoring: After actions are implemented, re-score O and D to compute the post-mitigation RPN — see Evaluate Post-Mitigation RPN
Sources
Sources
DFMEA risksheet configuration (
DFMEATemplate/risksheet.json), failure mode severity enumeration (failureModeSeverity-enum.xml), occurrence enumeration (fmOccurrence-enum.xml), detection enumeration (detection-enum.xml), failure mode custom fields (failureMode-custom-fields.xml), DFMEA top panel formulas (DFMEATemplate/risksheetTopPanel.vm).